Implantable infusion devices for delivering infusates allow a user to remain mobile while receiving therapy provided by the device. Modern implantable infusion devices commonly have a rigid housing that maintains a collapsible infusate reservoir. The housing includes a needle-penetrable septum that covers a infusate reservoir inlet port. A flow passage is provided between the infusate reservoir and an exterior surface of the device for delivery of infusate.
As with most implantable devices, it is desirable to minimize the size of implantable infusion devices to reduce inconvenience and irritation to a user. Advanced electronic circuits reduce the size of internal circuitry controlling implantable devices, and can reduce the size of power sources required to power the device. While circuitry advances have helped to minimize the size of implantable infusion devices, other components of the devices are often size-constraining. For example, the infusate reservoir inlet ports on implantable infusion devices may dictate the thickness of the device when they are stacked on top of the reservoir.
Reducing the size of the infusate reservoir inlet port faces certain constraints. When refilling an implantable infusion device, the septum covering the infusate reservoir inlet port is pierced with a needle and the infusate delivered to the infusate reservoir via the needle. The port dimensions perpendicular to the needle insertion direction must be large enough to ensure that the needle target can be located easily and the needle can be inserted without requiring multiple attempts at access. Also, the port must be deep enough parallel to the needle insertion direction to prevent the outlet of the needle from inadvertently coming out of the port during refilling and in turn delivering infusate to tissue surrounding the pump. Also, the thickness of the septum in the dimension along to the needle insertion direction must be great enough to ensure that a reliable seal is formed throughout the service life of the pump. Accordingly, modifications are constrained that reduce the height of the infusate reservoir inlet port and septum, thereby reducing the size of the implantable infusion device.
Furthermore, while it is desirable to minimize the overall size of an implantable infusion device, it is also desirable to maximize the size of the infusate reservoir relative to the total size of the implantable infusion device. Implantable infusion devices are generally refilled periodically via an infusate reservoir inlet port. While the refilling procedure is often minimally invasive for a user, it often requires a user to visit a healthcare facility or employ the services of a healthcare provider. Thus, if the size of the infusate reservoir is maximized, fewer refills would be necessary unless the drug stability is the factor limiting the period between refills. Smaller implantable infusion devices may be used, while minimizing the refill frequency, if the volumetric efficiency of the device is high. The volumetric efficiency refers to the total infusate capacity of the implantable infusion device compared to the total volume of the device.
Methods have been proposed to reduce the thickness of implantable infusion devices. U.S. Pat. No. 5,443,450 has a doughnut-shaped reservoir within a housing. The housing has a bulge near the top plate that allows the reservoir bellows to fully extend, thereby maximizing the capacity of the reservoir and minimizing the required thickness of the device. In U.S. Pat. Nos. 6,280,416 and 6,652,510 expandable polymeric bags and flexible diaphragms, respectively, are used to form infusate reservoirs, minimizing the required thickness of an associated implantable infusion device. Additionally, although all commercially available drug pumps utilize a central refill port, some designs have been proposed that place the fill port on the periphery of the pump. However, placement of the fill port on the periphery creates safety and other problems, because clinicians expect the fill port to be in the approximate center of the pump.
In some conventional solutions, a propellant gas such as a hydrocarbon, hydrofluorocarbon, chlorofluorocarbon, and/or similar compound, is used to pressurize an infusate reservoir. A reservoir may be positively pressurized with a propellant so that the reservoir pressure is greater than ambient pressure, or negatively pressured with a propellant so that the reservoir pressure is less than ambient pressure. Mechanical pressurization means may also be used. When propellants are used, the propellant chamber must be constructed of a material and in a manner that will inhibit the propellant from diffusing out of the propellant chamber. However, most polymeric materials have a measurable permeability that would allow propellant in vapor form to escape from the propellant chamber. Thus, the longevity of an implantable infusion device using propellant gas may be limited when polymeric materials are used to contain the propellant. Also, with the exception of the pump described in U.S. Pat. No. 5,443,450, devices using conventional reservoir shapes do not allow a port to protrude into space otherwise traversed by a collapsing or expanding reservoir, thereby requiring that the height of these devices be at least equal to the infusate reservoir inlet port height plus the reservoir thickness and increasing the overall height of those devices.